A great number of sensors are utilized for monitoring the properties of a media. Thus, for example, in the case of monitoring water quality in distribution networks, especially drinking water, at least one of the following parameters are monitored: Temperature; flow, e.g. flow rate; pressure; pH value; redox potential; oxygen, ammonium, phosphate, nitrate, nitrite and chlorine content; global parameters, especially chemical, and/or biochemical, oxygen demand; conductivity; and (dissolved) organic ingredients, especially total (dissolved) carbon and turbidity. In such case, so-called multiparameter sensors can be used. Multiparameter sensors are sensors that are able (simultaneously) to measure more than one process variable.
A distribution network in the sense of this invention is to be understood to include a (pipe-)line, silo, tank, container, water network or gas network.
Ideally, the sensors measure over a longer period of time, at least one year, without requiring maintenance and calibration. Of course, the measurements must be accurate over this period of time.
Sensors for measuring pH value, redox potential, oxygen and chlorine utilize predominantly electrochemical methods, especially potentiometric and amperometric methods. A problem at which the invention is directed will be explained here with reference to potentiometric sensors. The invention is, however, not limited to potentiometric sensors, but generally relates to sensors for use in electrochemical measuring systems. It is furthermore an option to apply the ideas of the invention to non-electrochemical sensors.
Potentiometric sensors have at least one measuring electrode and a reference electrode, between which a potential difference is determined. The potential difference is, in such case, dependent on the activity or the concentration of the substance being measured; in the case of a pH measurement, for example, dependent on the activity of the hydrogen ion H+, which, in the case of a pH measurement of a dilute solution, corresponds essentially to the proton concentration.
Besides systems comprising a measuring electrode and a reference electrode, which are introduced separately into a medium, especially in the field of pH measurement, so called single-rod measuring chains are also known, which combine measuring electrode and reference electrode in one probe. These probes comprise, as a rule, two concentrically arranged tubes, wherein the low resistance reference half cell is arranged in the outer tube, and simultaneously serves for shielding the high resistance pH half cell. Besides the described embodiment, there exist still other forms of embodiment of electrochemical sensors in thick film or thin film technology.
In all cases, at least one of the electrodes is immersed in an electrolyte, which has electrolytic contact with the medium via a liquid junction. Typically, silver/silver chloride electrodes are made use of in an electrolyte of defined chloride ion activity, as a rule 3 molar.
Especially the electrolyte of the reference electrode depletes with time via the liquid junction. This leads to a continuous drift in the measured values, and the measuring becomes inaccurate. Because of electrolyte depletion, the measured value drifts by, for instance, 0.1-0.2 mV per day. Disturbing diffusion and streaming potentials can also form, which likewise corrupt the measuring.